Reporting API v2: 400 error on agent utilization 30-min interval query

Trying to pull agent utilization metrics (tHandle, tAcw, tHold) broken down by 30-minute intervals via the Reporting API v2. The query structure looks right based on the docs but keeps returning a 400 Bad Request.

GET /api/v2/analytics/agents/utilization/query
{
 "interval": "PT30M",
 "select": ["tHandle", "tAcw", "tHold"],
 "groupBy": ["interval"]
}

Error response says Invalid interval format. What’s the exact syntax needed for the interval field?